[S390] standardize Kbuild rules

Introducing this Kbuild file allow us to:

make arch/s390/

And thus building all the core part of s390.
Same as on other architectures.

Signed-off-by: Heiko Carstens <[e-mail]>
Signed-off-by: Martin Schwidefsky <[e-mail]>

arch/s390/Kbuild

···
+obj-y += kernel/
+obj-y += mm/
+obj-y += crypto/
+obj-y += appldata/
+obj-y += hypfs/
+obj-y += kvm/

arch/s390/Makefile

···
 head-y		+= arch/s390/kernel/$(if $(CONFIG_64BIT),head64.o,head31.o)
 head-y		+= arch/s390/kernel/init_task.o
 
-core-y		+= arch/s390/mm/ arch/s390/kernel/ arch/s390/crypto/ \
-		   arch/s390/appldata/ arch/s390/hypfs/ arch/s390/kvm/
+# See arch/s390/Kbuild for content of core part of the kernel
+core-y		+= arch/s390/
 
 libs-y		+= arch/s390/lib/
 drivers-y	+= drivers/s390/